Add exercise statements and solutions notebooks

[jhlegarreta]

The introduction and DTI episodes have each an exercise so that learners can demonstrate the acquired skills; however, the exercises are only written in the Markdown files, and they are missing from the Jupyter notebooks.

The exercise instructions should be added to the notebooks and the solutions should be included in the solution notebooks.

[kaitj]

Can also add the CSD exercise to this list now. Going to wait until we finalize the exercise in #117 before adding to notebooks.

[kaitj]

DTI notebook has been taken care of, will also update CSD later this week.

I am leaving the intro notebook as those have presumably bigger changes coming before May.